In the United States, approximately 2.4 million burn injuries are reported per year. Many patients with major burns involving at least 25% of their total body surface die. The transplantation of large patches of skin would save their lives. However, allogeneic skin grafts are invariably rejected in an acute fashion. We have identified a treatment that can achieve tolerance i.e. graft acceptance in the absence of immunosuppression to skin allografts when they are vascularized at the time of transplantation. The objective of this proposal is to identify the mechanisms by which vascularization affects the immune response to skin allografts and promote long-term survival of these transplants. To address these issues, we propose the following aims: Specific aim 1. Investigate how vascularization affects the physiology of the immune response to skin allografts We will study the effects of skin graft vascularization of the T cell response (direct and indirect alloresponses by CD4+ and CD8+ T cells), the B cell response (alloantibody production), the trafficking of graft donor APCs (passenger leukocytes) to recipient's lymphoid organs and the migration of recipient's T cells including FoxP3+ regulatory T cells to the skin graft. Specific aim 2. Investigate the mechanisms underlying long-term survival of vascularized skin allografts via costimulation blockade. We will investigate the role of regulatory T cells in long-term survival of vascularized skin allografts in recipients treated with anti-CD40L antibodies. We will investigate whether mice transplanted with a vascularized skin and treated with anti-CD40L antibodies develop true donor-specific tolerance. If successful, this research will set the path for the development of immune-based strategies allowing the storage and successful transplantation of large patches of allogeneic skin for patients with burns and for reconstructive surgery (including face transplants). In addition, successful tolerance to skin allografts could be used to achieve systemic tolerance to donor antigens prior to transplantation with solid organ transplants such as kidneys, hearts or lungs. In addition to its implications in transplantation biology, this research may lead to a better understanding on the mechanisms by which vascularization of tumors affects the immune response.